1. Field of the Invention
This invention relates generally to control strategies for management of drivetrain torque in electric vehicles and partial electric vehicles. More specifically, it relates to a sub-strategy for management of vehicle rollback.
2. Background Information
A wheeled automotive vehicle may comprise one or more electronic modules that control various aspects of powertrain operation. Certain vehicles have a powertrain that includes a rotary electric machine. A vehicle that comprises such as a machine as the sole prime mover is commonly referred to as an electric vehicle, and in such a vehicle, batteries or fuel cells are typical power supplies for the electric machine. Vehicles that include an electric machine in association with another prime mover, an internal combustion engine for example, are sometimes called partial electric vehicles or hybrid electric vehicles.
A powertrain control module (PCM) is a name that is sometimes given to an electronic module that processes certain data to control various aspects of powertrain operation. A rotary electric machine may be one device whose operation is under the control of a PCM. Sometimes the electric machine operates as a motor that makes a positive torque contribution to powertrain torque. At other times the electric machine operates as a generator that makes a negative torque contribution to powertrain torque. Positive torque contribution from the electric machine may appear as traction torque delivered through a drivetrain of the vehicle to at least some of the wheels to propel the vehicle. Negative torque contribution from the electric machine may be used to impose braking torque on the drivetrain to brake the vehicle. In a hybrid electric vehicle, positive and negative torque contributions from the electric machine may also be used to smooth torque fluctuations due to combustion events in an associated combustion engine.
When an electric machine imposes braking torque on the drivetrain to brake the vehicle, it is operating as an electric generator. Generated electricity may be used advantageously to regenerate an electric power supply such as a battery or fuel cell. Hence, such braking is commonly referred to as regenerative braking, or sometimes simply regen for short. A vehicle that possesses regenerative braking capability typically does not rely exclusively on such braking for the vehicle service brakes. While some energy recovery is made possible by regenerative braking, it is inappropriate at certain times to invoke regenerative braking. For example, the state of charge (SOC) of a battery, or battery bank, may be such that regenerative electric current from the electric machine should not be fed, either in whole or in part, to the battery or bank. In the absence of a suitable sink for such electric current, an alternate braking means is needed.
Hence, both full and partial electric vehicles employ some form of mechanical brakes, such as friction brakes at individual wheels. Mechanical friction brakes may be hydraulic-, pneumatic-, or electric-operated. It is known to use an electronic brake controller or brake control unit (BCU) to apply relative proportions of regenerative braking and friction braking when braking is called for.
It may be considered desirable, in certain like driving situations, for certain operational characteristics of an electric vehicle to mimic those of a vehicle powered by an internal combustion engine acting through a drivetrain that has an automatic transmission. For example, when an internal combustion engine powered vehicle is operated on a horizontal surface with the automatic transmission in a forward or reverse drive gear, and without either the accelerator pedal or the brake pedal being depressed, it may be deemed desirable for the idling engine to deliver enough torque through the drivetrain to cause the vehicle to accelerate in the direction of the selected gear from zero speed to some calibratable, yet fairly small, running speed at which the torque is just sufficient to maintain that speed. This is often referred to a vehicle creep.
Application of the friction brake opposes vehicle creep. If the vehicle is on an inclined, rather than a horizontal, surface, the amount of inclination will influence vehicle creep. If the degree of inclination were to increase, creep speed would decrease, eventually reaching zero speed at some particular grade, corresponding to holding the grade. Beyond that, the torque would be insufficient to maintain even zero speed, and the vehicle would begin to roll down the grade in the opposite direction from the direction of the selected gear. This is referred to a rollback. A driver of the vehicle may see fit to apply friction brakes at any particular time while the vehicle transmission is in a forward or reverse drive with the engine idling, and is especially likely to do so to counter an incipient rollback on a grade. But some rollback may occur before the friction brakes are effective to stop it. Moreover, the driver may depress the accelerator pedal to attempt to arrest the rollback, and if both accelerator pedal and brake pedal are applied more or less concurrently, the vehicle is said to be two-pedaled. Such two-pedaling may create conflicting commands that, unless properly resolved, may interfere with prompt correction of rollback and/or create undesired effects such as powertrain torque spikes, or shudder.
Although certain rotary electric motors are capable of bi-directional operation, their inherent characteristics, and those of on-board energy storage media for powering them (e.g. batteries), may cause certain general torque control strategies not to produce desired results during vehicle rollback. For example, internal energy requirements of certain electric induction motors render them incapable of producing regenerative energy at low speeds such as those occurring during rollback. Hence, when vehicle speed is zero, meaning the vehicle is not in motion, the motor control system assumes a state for delivering electric current to the motor, rather an opposite state. When incipient rollback is sensed by the system, the current flow to the motor will be increased in an attempt to counteract the rollback. For any of various reasons however, rollback is apt not to be instantaneously countered, in which event the vehicle will actually begin rolling back. At some rollback speed, which is a function of motor and drivetrain characteristics, the motor will inherently generate counter EMF sufficient to create an opposing electric current sufficient to cancel the current flow from the on-board storage medium. At greater speeds, the direction of current flow reverses, initiating regenerative current. Should the state of the on-board energy storage medium be inappropriate for sinking this regenerative current, an undesirable condition may ensue.
It is toward avoiding undesired conditions, such as that one, that the present invention is directed.
Such conditions may be even further complicated by certain torque management strategies at and near zero vehicle speed. For example, certain torque management strategies may choose to reduce energy losses by canceling or partially canceling drive torque with braking torque in lieu of allowing the drive torque and brake torque to be applied simultaneously. In that case, torque steps or spiking may occur, due to torque sign changes, when transitioning through zero mph.
A preliminary novelty search developed the following U.S. Patents as evidencing the state of the art: U.S. Pat. Nos. 4,629,043; 5,376,869; 5,467,275; 5,757,153; and 5,984,034.